Long Term Evolution (LTE), which is a mobile communication standard, provides features that improve spectrum efficiency and reduce latency as compared to conventional communication standards. LTE is thus expected to be a promising standard that will support evolution of future mobile communication business.
LTE employs single carrier-frequency division multiplexing access (SC-FDMA) as the uplink modulation scheme. In SC-FDMA, the minimum unit for forming an information symbol in the frequency domain is one subcarrier. A set of 12 subcarriers is called one resource block (hereinafter, referred to as “RB”), and frequency resources for each uplink channel are assigned in units of RBs.
Physical uplink control channel (PUCCH) format 2, PUCCH format 1, physical random access channel (PRACH) and physical uplink shared channel (PUSCH) are assigned as uplink channels.
A scheduler in a base station apparatus determines the frequency position, the subframe timing and the number of RBs used for each channel (see, NPL 1, for example).
When observed in the time direction, a transmission of each channel is allocated in a unit called a subframe. Each subframe consists of 14 SC-FDMA symbols for normal cyclic prefix (CP) and consists of 12 SC-FDMA symbols for extended CP. PUSCH and PUCCH are scheduled in units of subframes. PRACH is used to transmit initial connection signals called a preamble, and allocation of a PRACH transmission is dynamically set within a range of one to three subframes in accordance with a cell radius and/or the like of a base station apparatus.
In recent years, development of small base station apparatuses called pico cells or Home eNBs (hereinafter, abbreviated as “HeNB”) has been carried out for the purpose of covering dead zones for mobile phones.
HeNBs are supposedly installed indoors in most cases and are said to have a capacity of several terminals in general and several tens of terminals at most. Moreover, a cell formed by a HeNB is smaller than a cell formed by a conventional macro base station apparatus (hereinafter, referred to as “MeNB”). In addition, all communication terminal apparatuses served by a HeNB or a MeNB communicate with the corresponding HeNB or MeNB, using the SC-FDMA scheme having the frame structure described above.
Conventional MeNBs are installed in accordance with a base station design appropriately developed by a communication carrier in advance. Thus, inter-cell interference would not be a significant problem with MeNBs. Meanwhile, HeNBs can be installed anywhere by end users, so that interference with a MeNB will be a significant problem. In particular, interference to a preamble in uplink random access causes a significant problem in an initial connection attempt made by a mobile station served by a HeNB (hereinafter, such a mobile station is referred to as “HUE”).
The 3rd Generation Partnership Project (3GPP), which is a standardization body for mobile communication standards, has proposed various techniques as typified by a technique described in NPL 2. NPL 2 discloses a technique that reserves part of resources of a MeNB for a preamble for a HeNB, thereby preventing preambles transmitted from communication terminal apparatuses served by the respective base station apparatuses from interfering with each other.